1. Field of the Invention
This invention relates to earth boring bits, lubricated with a system which includes a hydrostatic pressure compensator to balance the internal pressure of the lubricant inside the bit with the hydrostatic pressure of a liquid drilling fluid that surrounds the bit during drilling. In this combination, the specific improvement relates to the seal and bearing assembly between each cutter and bearing shaft.
2. Background Information
The preferred embodiment in the above U.S. Pat. No. 4,516,641 utilizes a pair of rigid face seals positioned in a seal groove, including a pair of resilient energizer rings, preferably of the O-ring type. The dimensional relationships of the sealing components and the groove result in greater axial movement of the rigid face seals than the associated cutter.
As a bit rotated during drilling, its cutters move axially, or with a rocking motion, on the bearing shafts because of the clearances and normal manufacturing tolerances. Some clearances are necessary to assemble the cutters on the shafts. Axial and radial cutter movements which results from the clearance between cutter and shaft causes rapid pressure variations in the lubricant, or more accurately, volume changes in the lubricant in the vicinity of each seal. In the preferred embodiment of the above U.S. Pat. No. 4,516,641, the rigid face seals may move axially a distance greater than the axial cutter movement by a ratio of about 1.88 to one.
The disclosure in the second of the above U.S. Pat. No. 4,666,001 teaches the positioning of the seal groove and seal assembly in relation to the bearing surface such that rigid face seal movement is decreased relative to axial cutter movement during drilling to enhance seal life. Preferably the ratio of rigid ring movement to axial cutter movement is substantially one-half to one.
A two piece bearing construction is disclosed in U.S. Pat. No. 4,600,064, "Earth Boring Bit With Bearing Sleeve", July 15, 1986. An internally tapered and threaded bearing sleeve is made up on a mating, externally threaded bearing lug. The mouth of the sleeve engages a shoulder on the base region of the the bearing lug and has a selected radial thickness such that the sleeve may be made up to a selected torque. Also, the bearing sleeve has a length greater than that of the threaded portion to define a thick walled inner end region to receive a resilient retainer ring in a groove that provides a selected section over the threads on the interior of the sleeve.